Bidirectional 1+1 APS example

Table: 1+1 APS for Bidirectional Mode – Actions Taken outlines the steps that the bidirectional APS process will go through during a typical automatic switching event. The example is read row by row, from left to right, to provide the complete process of the bidirectional switching event.

Table: 1+1 APS for Bidirectional Mode – Actions Taken
Status	APS Commands Sent in K1 and K2 Bytes on Protection Line		Action
	B to A	A to B	At Site B	At Site A
No failure (protection line is not in use)	‟No request”	‟No request”	No action	No action
Working line degraded in direction A to B	‟SD” on working channel 1	‟No request”	Failure detected, notify A and switch to protection line	No action
Site A receives SD failure condition	Same	‟Reverse request”	No action	Remote failure detected, acknowledge and switch to protection line
Site B receives ‟Reverse request”	Same	Same	No action	No action